With trade and commerce of energy resources and ecological threats stemming from their use impacting geopolitical stability, the reality is that humankind is at a crossroads, which in turn is driving an imperative to develop alternative energy sources. Humankind's tremendous industrial and technological progress over the last two centuries has been driven by the natural abundance and availability of fossil fuels. As those reserves deplete, the prudent course of action would be to develop other readily available fuel sources. Currently, 15 terawatts (1.5×1013 Watts) of energy are consumed worldwide annually, with fossil fuels providing 86% of that energy. The categories of energy forms used each year are 40% petroleum, 23% natural gas, 23% coal, 8% nuclear, and 6% from renewable sources. The projected years left in reserves at the current usage levels are estimated at 47 years for petroleum reserves, 60 years for natural gas reserves and 131 years for coal reserves [1].
Renewable energy sources include solar, wind, hydroelectric and geothermal options. Solar energy is accessed and utilized in several ways. An important way is the use of silicon based solar cells that produces electric current. A recent important breakthrough in the research of inexpensive materials for energy applications is the use of a silicon sheet coated with a cobalt based catalyst on one side and a nickel-molybdenum-zinc alloy layer on the opposite side [2]. This device, named the artificial leaf, produces notable amounts of hydrogen and oxygen when submerged in water and illuminated. Another major current research effort involves the use of TiO2 exposed to UV radiation which splits water into hydrogen and oxygen [3]. Water-splitting TiO2 is mostly used in fuel cells that generate electric current from the oxidation of hydrogen. The challenge over the last three decades has been to develop novel materials to perform water splitting with energy from the visible light region of the electro-magnetic spectrum. The use of nanoparticles alone or in composite materials with TiO2 show promise for improving this exploitation of solar energy [4].
Another research area toward the goal of accessing solar energy, but with a somewhat different outcome is the production of hydrogen as an alternative fuel source by utilizing biomolecules. Some researchers using biological systems to produce hydrogen, utilize one or both of two enzymes of hydrogenases and nitrogenases found in certain microorganisms. Developing ways to utilize and adapt this hydrogen generating ability of these enzyme systems can be grouped in three broad approaches: using an enzyme itself; forming a hybrid between different enzyme components and a synthetic material, or by synthesizing a biomimetic analog of the biological catalyst.
Some research efforts using biomolecules involve the hydrogenases and nitrogenases with the goal of evolving hydrogen. There are several published methods that have photocatalytically generated hydrogen. For example, self-assembled complexes between cadmium telluride nanocrystals and Fe—Fe hydrogenase from Clostridium acetobutylicum when illuminated with a visible light source have produced hydrogen with ascorbic acid serving as an electron donor [5]. A NiFeSe-hydrogenase from Desulfomicrobium baculatum complexed with ruthenium dye-sensitized TiO2 was illuminated either with a tungsten halogen lamp or with sunlight and was effective for production of hydrogen [6]. An altered molybdenum-iron protein (MoFe protein), component I protein of nitrogenase, was complexed with Ru(bypy)2 near the catalytic site. When the complex was illuminated with a xenon/mercury lamp and provided with the substrates of protons or acetylene, the system produced the corresponding reduced products of hydrogen and ethylene [7].
However, the above systems have only displayed limited capability and efficiency in producing hydrogen (H2). In particular, the longest lasting hydrogen production system utilizing inexpensive nanoparticles complexed to a nitrogenase or hydrogenase in published research to date is only 4 hours. Thus, a need exists for improved systems and methods for producing hydrogen, in particular over an extended or prolonged period of time (e.g., for at least 5 hours, at least 10 hours, at least 50 hours, at least 90 hours, or at least 100 hours, or longer).